1. Field of the Invention
The present invention relates to a method and an apparatus for evaluating the distribution of various properties of an optical fiber along its longitudinal direction, such as optical loss in the optical fiber (optical attenuation), the relative refractive index difference between the core and cladding, core diameter, variation in stresses applied to the optical fiber and influence of temperature change on the optical fiber.
2. Prior Art
As means for determining the distribution of the loss of an optical fiber along its longitudinal direction, there has been proposed optical time domain reflectometry (hereinafter referred to as "OTDR") in which Rayleigh back-scattered light generated in an optical fiber is detected (see, for instance, M K, Barnoski et at., Appl. Opt., 1977, Vol. 16, pp. 2375-2379, "Optical time domain reflectometer"). This method has been improved over 10 years since it was proposed and at present becomes an indispensable technique for the installation of an optical communication network and for the maintenance thereof. There has also been proposed an optical frequency-domain reflectometry (hereinafter referred to as "OFDR") in which Rayleigh back-scattered light is analyzed in the frequency domain, while the OTDR method analyzes it in time domain (see, for instance, F P Kapron et al., Tech. Digest of IOOC'81, 1981, p. 106, "Aspect of optical frequency-domain reflectometry"). These OTDR and OFDR methods have almost reached the stage of completion.
However, since the strength of the back-scattered light is very weak, it is difficult to increase the precision of its measurement even if the signal to noise ratio is improved with a device such as an averaging device.
A typical value of measurable one-way optical loss of commercially available OTDR apparatuses is about 20 dB when the input optical power within the test fiber is about 1 mW; the distance resolution is 100 m (optical pulse width=1 microsecond) and the average processing time is on the order of one minute. In order to extend the dynamic range (i,e., measurable maximum optical attenuation of an optical fiber), it is required to employ large-scale high power lasers, represented by YAG lasers, and/or to make a sacrifice of the spatial resolution and/or measurement time.
Furthermore, the amount of light reflected at the light input endface of the test fiber or connectors in the optical transmission line is greater than the Rayleigh scattered light by 3 or 4 orders of magnitude and this results in saturation of a photo detector. For this reason, a back-scattered light cannot be measured over a certain distance ahead of the reflection point and thus a so-called dead zone is formed.
An attempt for determining stresses in an optical fiber was reported by M. C. Farries et ak., where they observed Raman scattering amplification in an optical fiber when two lights having different wavelengths are counterpropagated in the fiber (Tech Digest in OFS (Symposium of Optical Fiber Sensing) '84, pp. 121-132, entitled "DISTRIBUTED SENSING USING STIMULATED RAMAN INTERACTION IN A MONOMODE OPTICAL FIBRE") by M. C. Farries et al. Although, in this report, the signal waveform change due to stress is recognized, there is no quantitative relationship between the stress and the waveform change and it failed to locate the position at which the stress is applied. This is because the technique in the above mentioned report utilizes the polarization effect on Raman optical amplification gain, but it is difficult to interpret a double-pass integrated polarization state in terms of the stress applied to the fiber.